penn-ohio chapter training september 20, 2012. harmonic confidential introduction review of optical...
TRANSCRIPT
Harmonic Confidential
AGENDA
Introduction
Review of optical components and their impact on system performance
Direct fed 1310 TX
Long haul 1550 TX
1550nm Broadcast / narrowcast
Full band TX (O-band, C-band, EM, EAM)
Summary
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Typical networking link
O/T
O/RRFin RFout
Splices
Connectors
• Transmitter• fiber• splice/connector• Optical amplifier• Receiver
Harmonic Confidential
Typical networking link
O/T
O/RRFin RFout
Splices
Connectors
• Transmitter• fiber• splice/connector• Optical amplifier• Receiver
Performance is going to depend on:RF drive level, launched power, laser RIN, number of channels, reflection parameters,EDFA noise figure, EDFA input power, received power, receiver quantum efficiency, receiver Thermal noise, Input performance, receiver output power, optical modulation index, number of wavelength in the system, flatness of the filters, transmitter linearization quality, splice quantity, SBS parameters, laser chirp, type of fiber, connector cleanliness, ……
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Single mode fiber characteristic
Attenuation−1310 nm: < 0.35 dB/km−Minimum loss near 1550 nm: < 0.22 dB/km−Standard design value @ 1550 nm: 0.25 dB/km
Dispersion−Dispersion: Traveling speed of a lightwave in a medium
varies with wavelength −Dispersion parameter for SMF-28 fiber
• Zero near 1310 nm• +17 [ps/(nm*km)] @ 1550 nm
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Attenuation versus wavelength
0.0
0.50
1.0
1.5
2.0
2.5
800 1000 1200 1400 1600
Atte
nuat
ion
(dB
/km
)
Wavelength, nm
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Dispersion characteristic of single mode fiber
Wavelength, nm
-120
-100
-80
-60
-40
-20
0
20
40
800 1000 1200 1400 1600
Dis
pe
rsio
n [p
s/(n
m*
km)]
Standard
Dispersion Shift
Dispersion Flat
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Single mode lasers: Key parameters
Linewidth
RIN noise
Wavelength
• Center wavelength (nm)
• Power (dBm or mW)
(0dBm=1mW, 10dBm=10mW, 20dBm=100mW)
• Linewidth (typical MHz)
• RIN noise (typical 155dB/Hz)
• Chirp (MHz/mA)
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Distributed FeedBack laser (DFB)(usually uses an Isolator)
Uncooled DFB−No temperature control Wavelength varies with
temperature−Cheaper−Used for non-WDM application or CWDM application
Cooled DFB−Uses a TEC to keep the temperature constant.−Wavelength stays constant with outside temperature−Used for DWDM−More expensive.
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Optical transmitter: Intensity modulation
Directly modulated
Externally modulated
Laser
RFPre-distortion
Bias circuit
Optical OutputRF Input
Laser Modulator
RF Pre-distortion
Bias circuit
Optical OutputRF Input
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Directly modulated: L-I curve
Ith
Bias Point
Ligh
t Int
ensi
ty
RF Drive current
L I Curve
DC Bias Current
• curve is non linear• Wavelength depends on current chirp
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Optical Modulation Index (OMI)
Time
Op
tica
l Lev
el
(Po
we
r)
Transmitter DC output power, P0
Modulation index per single channel, msingle
ch. =
PPP0
(msingle ch. 100 % , otherwise clipping)
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Composite modulation index
For a multichannel system, the RF carriers are uncorrelated and the effective modulation index is the root mean square (rms) sum of the indexes of each channels.
Composite OMI= N1/2x (OMI/ch)
where N is the total channel number, msingle is the modulation index of a single channel.Total RMS modulation should be limited to 25-30%.
Example: for 80a, OMI per channel= 3.5%
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Performance vs. received power
Pin
RIN limited (flat)Shot noiseLimited (1dB/dB)
Thermal noiseLimited (2dB/dB)
The higher the received power the better the CNR
Not applicable to direct-mod
1550nm FS trransmitters
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Performance vs. OMI with analog channel only
OMI
Per
form
ance
CNR increases 1dB per dB
CSO degrades 1dB per dB
CTB degrades 2dB per dB
The higher the OMI the better the CNR but the worst the distortion
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Performance vs. OMI with analog + QAM channels
OMI
Per
form
ance
CNR has an optimum point
CSO degrades 1dB per dB
CTB degrades 2dB per dB
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Chirp in Directly Modulated Systems
I
Current
Chirp + dispersion creates distortion- No full band directly modulated system at 1550nm only at 1310nm- Externally modulated system at 1550nm for analog
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Setting up your 1310 Link
Initial setup− Verify RF input is the correct level.− RF input should be flat.− Note: Factory Settings (Harmonic)
• 80 unmodulated carriers 45 to 550 MHz.• Above 550 is 450 MHz digital -6db down from analog.• RF input level is 15dbmv.• If the channel load is different adjust RF input accordingly.
− Run Auto Setup (Harmonic)− Fine Tune the transmitter by manually adjusting the internal RF
pad.
Periodically− Verify RF input is flat and the correct level.− Verify delta between the analog and digital channels.− If the transmitter is in MGC and the channel load has changed
re-optimize the RF input to the laser.
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1550nm Transmitter Broadcast and Long-Haul Applications
Externally modulated. Transmitter
Rx
Rx
EDFA Optical
Amplifier
Optical Receiver
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Optical transmitter: Intensity modulation
Directly modulated
Externally modulated
Laser
RFPre-distortion
Bias circuit
Optical OutputRF Input
Laser Modulator
RF Pre-distortion
Bias circuit
OpticalOutputRF Input
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Stimulated Brillouin Scattering
Non-linear effect in fiber that limits the amount of light that can be launched into fiber to about 7dBm per 20MHz BW) Special technique are used to limit the effect of SBS in externally modulated system allow launch of 17dBm with one wavelength Beating between incoming & reflected laser beams introduce additional CSO & CTB distortions
Pin
Pout
Prefl
Ptrans
Acoustic wave
light
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Setting up your 1550 Link
Initial setup− Verify RF input level of 18 dBmV (Harmonic) − RF input should be flat.− Turn Switch to Factory Settings in AGC (Harmonic)− Note: Factory settings
- RF input 18dBmv- MGC- 80 NTSC Channels- Set pilot pads accordingly.- Check for SBS and adjust accordingly.
SBS Adjustment (Harmonic)- Under Transmitter adjustments- Select Dual tone for links less than 85km. Select Single tone for
links longer than 85 km. In single tone max optical launch power is 14dBm. Adjust SBS 1 or SBS2 as necessary.
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BC/NC Architecture: Overview
Headend
1550-nm BC Tx
l1
l2
lN
Hub
NodesOptical filter
BCNC
NC
NC • Important parameters- Channel loading- link noise- Optical Rx power- Optical delta- Drive levels
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Well Served by this Solid Architecture (but …)
Good performance (>51 dB CNR) using fewer fibers
Good fiber reach (50 km or more)
Now possible to use O-Hubs instead of buildings
Some limitations starting to become apparent−Older narrowcast transmitters limited to 8 QAMs
−Newer transmitters support up to 50 QAMsCNR BC aloneCNR BC+NC
NC number of QAM
BER QAM
−Must decrease BC/NC optical delta
−Dual receivers offer advantage
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Setting up your BC/NC Link
1- Setup the BC transmitter at the right level (not overdriven)
2- Setup the optical delta between BC and NC. -10 for 64 QAM and -6 for 256 QAM.
3 -Adjust RF pad on NC TX to have the proper level for the QAM NC compared to the BC.
(1) (2) (3)
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Dual Receiver Option
Headend
1550-nm BC Tx
l1
l2
Hub
Nodes
Optical Filter
BCNC
NC
+
RF filter +RF combiner
• Removes the NC noise on the BC• Removes the BC beat term below the NC (if BC Tx is overdriven)• Optical delta is not so important anymore• Level of NC QAMs are adjusted in the node
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WDM Full Spectrum Transmitters
O-Band (1260nm – 1360nm) is older technology limited by Raman Crosstalk.
• Large wavelength separation causes a problem … trade off between number of wavelengths and launched power
Two competing technologies at C-Band (1530-1565nm)
• Low chirp laser sources such as external modulation or electro-absorbtion modulator (EAM)
• Widely available laser sources using newest predistortion technology to control dispersion
FS Transmitters offer segmentation options never before possible and have advantages over BC/NC architectures
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Full Spectrum Performance Considerations
Is it time to re-think our node input levels ??• Traditionally, we have targeted 0 dBm or higher• Modeling shows that levels of -5 to +3 dBm
offers flat MER performance with mostly QAM loading
RIN limited (flat)
Operating region,
traditional
Operating region,
DWDM 1550 nm
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The overall CNR of a fiber optic communication system from all the noise sources:
m Modulation Index Per Channelr Detector Responsivity [A / W], 1310nm: 0.85, 1550nm: 1.0Pr Detected Average Optical Power [W]
B Noise Equivalent Bandwidth, Video BW For TV system [Hz]q Electron Charge [Coulomb], 1.6 * 10-19
Ith Receiver Thermal Noise [A/Hz 0.5]
RIN Relative Intensity Noise [Hz-1] From Various Sources.
Signal
Relative Intensity Of Light
Shot Noise Thermal Noise
CNR of Optical Link
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Laser RIN - Typically Small ContributionEDFA Noise - Small or large depending on optical input power (per wavelength) into the EDFA and number of EDFAs in the link.Fiber Noise - Depends on the technology and fiber length. Large contribution with long fibers with SPL; small contribution with HLT and PWL.CIN (Intermodulation Noise) - Depends on QAM load, fiber length, technology,..Four Wave Mixing (FWM) - Depends on number of optical channels, wavelength separation between channels, optical power into fiber,…
IF link noise is dominated by RIN noise, then…CNR doesn’t improve much with increased received power
RIN noise behaves like this: 1dB increase of optical received power translates into 2dB increase in RF carrier level and 2dB increase in noise power translating into RIN generated CNR independent of received power
RIN Sources
Raising the node optical levels may actually decrease the CNR/MER because you have increased the RIN as a result of increased power in the fiber
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Full Spectrum Performance Considerations
Is it time to re-think our node input levels ??• Traditionally, we have targeted 0 dBm or higher• Modeling shows that levels of -5 to –3 dBm
offers optimum performance
What should the performance target be for MER ??• Today, operators strive for 38-39 dB MER• Studies suggest that with all QAM networks,
35-36 dB MER offers great performance and plenty of margin
• Some say that BER is a better performance indicator